This invention relates generally to a novel structure for a giant magnetoresistance sensor suitable for high density data applications and to systems which incorporate such sensors. In addition, this invention finds utility in any other application in which magnetic field sensing is desired.
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disc drive) incorporating rotating magnetic discs is commonly used for storing data in magnetic form on the disc surfaces. Data are recorded on concentric, radially spaced tracks on the disc surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disc surfaces.
In high capacity disc drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their ability to read data from a surface of a disc at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer or layers (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
FIG. 1(a) illustrates a simple, unpinned GMR sensor 100. The simple GMR sensor consists of two magnetic layers 103 and 105 separated by a nonmagnetic spacer 104. A cap layer 106 covers one magnetic layer 105 and a buffer layer 102 is disposed under the other magnetic layer 103. The entire structure is deposited on a substrate 101. This simple unpinned GMR sensor 100 provides a limited GMR resulting in a relatively weak signal.
FIG. 1(b) illustrates the magnetization directions of the simple unpinned GMR sensor 100 with a bias current 110 flowing into the page. With current bias 110 the magnetization directions of the magnetic layers 105 and 103 are oriented mainly anti-parallel to each other as shown by the arrows.
FIG. 1(c) illustrates the magnetization directions of the simple unpinned GMR sensor 100 with a bias current 110 flowing into the page and an external magnetic field 111 applied. When a large enough external field 111 is applied, magnetization of the magnetic layers 105 and 103 will align with the field direction and the resistance will be low.
The sensors shown in FIGS. 1(a)-(c) are useful for applications such as magnetic field sensing. Simple unpinned GMR sensors have been used in bridge circuits, however, to operate successfully, i.e., provide a differential in resistance, one set of simple, unpinned GMR sensors must be either shielded or additionally biased. This additional shielding or biasing adds additional cost and complexity to the bridge circuit.
Therefore, there is a need for a magnetoresistive sensor that provides an increased GMR, resulting in a higher signal output. Also, there is a need for sensors that provide different field responses based on the current density applied to the sensor without requiring the additional complexity of shielding or biasing.
According to a first aspect of the present invention there is provided a magnetoresitive (GMR) sensor including a substrate and a first trilayer disposed on the substrate. A first spacer layer is disposed on the first trilayer. A first magnetic layer is disposed on the first spacer. A second spacer layer is disposed on the first magnetic layer. A second magnetic layer is disposed on the second spacer layer. A third spacer layer is disposed on the second magnetic layer. A second trilayer is disposed on the third spacer layer and a cap layer is disposed on the second trilayer. The first and second trilayers include, a first ferromagnetic layer, a second ferromagnetic layer and an anti-parallel coupling layer disposed between and in contact with the first and second ferromagnetic layers.
According to another aspect of the present invention there is provided a magnetoresistive sensor device including a substrate and a first trilayer disposed on the substrate. A first spacer layer is disposed on the first trilayer. A first magnetic layer is disposed on the first spacer. A second spacer layer is disposed on the first magnetic layer. A second magnetic layer is disposed on the second spacer layer. A third spacer layer is disposed on the second magnetic layer. A second trilayer is disposed on the third spacer layer and a cap layer is disposed on the second trilayer. The first and second trilayers include, a first ferromagnetic layer, a second ferromagnetic layer and an anti-parallel coupling layer disposed between and in contact with the first and second ferromagnetic layers. The resistance of the magnetoresistive sensor is dependent on the magnitude of an applied bias current.
According to another aspect of the present invention there is provided a bridge circuit including a first pair of magnetoresistive structures coupled to first opposite nodes of a Wheatstone bridge and a second pair of magnetoresistive structures coupled to second opposite nodes of the Wheatstone bridge The first pair of magnetoresistive structures has a greater current density than the second pair of magnetoresistive structures when an external field is applied to the Wheatstone bridge.
According to another aspect of the present invention there is provided a disc drive system including a magnetic recording disc, a magnetoresitive sensor, an actuator for moving the magnetoresitive sensor across the magnetic recording disc and a detection circuitry electrically coupled to the magnetoresitive sensor for detecting changes in resistance of the magnetoresitive sensor caused by rotation of the magnetization axes of the first and second laminate layers in response to magnetic fields from the magnetically recorded data. The magnetoresistive sensor includes a substrate and a first trilayer disposed on the substrate. A first spacer layer is disposed on the first trilayer. A first magnetic layer is disposed on the first spacer. A second spacer layer is disposed on the first magnetic layer. A second magnetic layer is disposed on the second spacer layer. A third spacer layer is disposed on the second magnetic layer. A second trilayer is disposed on the third spacer layer and a cap layer is disposed on the second trilayer. The first and second trilayers include, a first ferromagnetic layer, a second ferromagnetic layer and an anti-parallel coupling layer disposed between and in contact with the first and second ferromagnetic layers.
According to another aspect of the present invention there is provided a an apparatus for measuring an external field applied across a Wheatstone bridge. The apparatus includes a four terminal electrical network (A, B, C, D) including a first resistor R1 connected between network terminals (A) and (B), a second resistor R2 connected between terminals (B) and (C), a third resistor R3 connected between the network terminals (C) and (D) and a fourth resistor R4 being connected across network terminals (A) and (D). The resistors R1 and R3 have a first current density when a field is applied across network terminals (A) and (C) and the resistors R2 and R4 have a second current density when the same field is applied across network terminals (A) and (C). The second current density is not equal to the first current density. The apparatus also includes means operatively coupled across the network terminals (B) and (D) for detecting a potential across the terminals (B) and (D).
The above, as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.